1. Field of the Invention
The present invention relates to a focus detection apparatus used for automatic focus detection, a method for controlling the same, and an image capturing apparatus that uses the focus detection apparatus.
2. Description of the Related Art
Conventionally, there are known to be image capturing apparatuses and the like that have an auto-focus (AF) function according to which a subject is automatically focused on by the position of an imaging lens being adjusted according to a detected focus state of the subject, which is detected with a focus state detection sensor having photoelectric converter elements. Furthermore, it is known that the charge accumulation period and output gain of a photoelectric converter included in the focus state detection sensor is controlled according to the brightness and contrast of the subject.
For example, Japanese Patent Laid-Open No. 2006-251777 discloses a focus state detection sensor that uses a line sensor composed of multiple photoelectric converters (pixels), in which the line sensor is divided into multiple areas and accumulation is stopped at a time when the difference between the maximum value and the minimum value of the pixel signal for each area (contrast) exceeds a target value.
Also, Japanese Patent Laid-Open No. 10-333021 discloses that a monitor sensor is arranged near a line sensor pair used for focus detection, and the accumulation (integration) period for the line sensor pair is controlled according to a signal from the monitor sensor.
However, with the configuration disclosed in Japanese Patent Laid-Open No. 2006-251777, since the charge resulting from the photoelectric conversion performed by the pixels is always transferred to an accumulation circuit, noise occurring in the accumulation circuit during accumulation is also accumulated similarly to the charge resulting from the photoelectric conversion. If the accumulation period is long, the amount of noise that occurs will also increase, and therefore there has been a risk that noise components will significantly influence the charge result, which will cause an error to occur in the focus detection result.
On the other hand, in Japanese Patent Laid-Open No. 10-333021, since accumulation control is performed based on the output of the monitor sensor provided separately from the line sensor pair, the charge resulting from photoelectric conversion performed by the pixels during accumulation does not need to be transferred to an accumulation circuit or a monitor circuit. Accordingly, it is possible to reduce the noise that occurs by resetting the accumulation circuit and the monitor circuit until the accumulation in the pixels ends.
However, in Japanese Patent Laid-Open No. 10-333021, the monitor sensor needs to be arranged near the line sensors and high-accuracy photometry needs to be performed using the monitor sensor, or suitable accumulation control is not possible. Also, if multiple line sensors are to be arranged, the monitor sensor limits the layout of the line sensors, and in order to perform high-accuracy photometry, the arrangement of the monitor sensor itself is also limited. Consequently, the chip area for the focus state detection sensor increases, which causes an increase in cost and prevents a reduction of the size of the optical apparatus that uses the focus state detection sensor.